Guidance on the Control of Odour and Noise from ... - Defra

2.1.5 Physical properties and odour perception
How an odour is perceived and its subsequent effects are not straightforward. The
human perception of odour is governed by complex relationships and its properties
need to be considered when assessing potential odour effects.
The perception of the intensity of odour in relation to the odour concentration is not a
linear but a logarithmic relationship. This means that if the concentration of an odour
increases tenfold, the perceived increase in intensity will be by a much smaller amount.
Interactions between mixtures of odorous compounds can also occur. These are known
as synergistic effects. An example of a synergistic effect is where one odorous
compound disguises or masks the presence of other compounds. As the odour
concentration reduces through dilution the nature of the odour may change as different
compounds dominate the effect. For example, certain emissions treated with incorrect
levels of odour counteracting agents has been observed to have distinctly different
odour characteristics at source than when diluted downwind. The odour intensity
experienced by an observer is, in general, not equivalent to the sum of the intensities of
the component odorous compounds. The perceived intensity may be greater or less
than the components depending on the synergistic effects of the compounds present.
Exposure to an odour can result in people becoming desensitised so that they can no
longer detect the odour even though the odorous chemical is constantly present in the
air. This is sometimes known as olfactory fatigue. For example, people working in an
environment with a persistent odour are often unaware of its presence and may not be
aware if the odour is having an impact on the surrounding community.
2.1.6 Factors that influence magnitude of an odour problem
Factors that influence the control of odour from commercial kitchens include:
Size of the cooking facility: This influences the intensity of the odour and
volume of ventilation air to be handled.
Type of food prepared: This affects the chemical constituents within the
ventilation air.
Type of cooking appliances used: This dictates the level of fat, water droplets and
temperature within the ventilation air.
2.1.7 Characteristics of different food types and cooking appliances
The odour and grease characteristics from a range of commercial kitchen types are
summarised in Table 2.2 (CIEH 1996 [4] ).
The grease and moisture characteristics anticipated from a range of kitchen appliances
are provided in Table 2.3 (LPS, 2003 [5] ).
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